Sleep and mood show complex relationships in depressed patients. Previous sleep deprivation studies addressing these relationships have not adequately controlled factors such as sleep timing, duration of prior wakefulness, and sleep stage composition, which may be important determinants of sleep and sleep deprivation effects on mood. There have been few studies which include age and sex-matched control subjects. A controlled study using two types of Partial Sleep Deprivation (PSD) will best answer the two questions which motivate this study: 1) Do sleep patterns and sleep-mood relationships differ for depressed patients and age-matched healthy control subjects during the course of sleep deprivation? and 2) Are sleep and sleep deprivation's effects on mood mediated primarily through sleep timing, through duration of prior wakefulness, or through sleep stage composition in depressed patients? Twenty patients with unipolar major depression and 20 age and sex-matched control subjects will each undergo two types of PSD: Sleep deprivation in the first half of the night (Early PSD; PSD-E) and sleep deprivation in the second half of the night (Late PSD; PSD-L). PSD-E and PSD-L will be performed in a counterbalanced order. Major outcome variables will include repeated EEG sleep measures of REM and Slow Wave Sleep (SWS) and self and observer mood ratings. This study will address the following specific hypotheses: 1. PSD-E will lead to a larger and earlier increase of REM sleep than PSD-L on sleep deprivation and recovery nights, but the two conditions will produce equal increases in Slow Wave Sleep (SWS). Depressed patients will have greater increases in REM, and smaller increases in SWS compared to healthy control subjects. 2. PSD-L will have a greater acute antidepressant effect than PSD-E. Neither form of PSD will have significant effects on mood in control subjects. 3. Mood improvement during PSD in depressed patients will be positively associated with: a) a delayed increase in the amount of REM sleep on sleep deprivation and recovery nights; b) earlier sleep hours; and c) the amount of SWS following sleep deprivation. Statistical analyses will include repeated-measures ANOVAs and both parametric and nonparametric correlational analyses.